In an optical transmitter using a conventional external modulator, generally, an optical signal of a single wavelength enters the modulator. Therefore, the optical transmitter has no function of processing an optical signal of plural wavelengths. In other words, there is no request for making the transmitter independent of a wavelength. However, in recent years, there has been a request for realization of a wavelength-independent optical transmitter, in relation to a request for cost reduction and improvement of flexibility of the optical communication system.
An optical transmitter using an electro absorption (EA) modulator can be easily integrated with other optical elements such as a light source and a semiconductor optical amplifier (SOA), and has an advantage in that the own driving voltage can be 2 to 3 volts as a sufficient value. On the other hand, this EA modulator has a disadvantage in that wavelength dependency is large, and requires a characteristic that the EA modulator is not dependent on the wavelength of an incident wave, that is, a control method of the EA modulator having wavelength independency.
For example, as a bias control method of an EA modulator, there is disclosed a control method of inputting an output of an LD light source controlled at a constant level to the EA modulator, taking out part of this output to obtain a modulation monitor light, and setting a ratio of this modulation monitor light to the back monitor light of the LD light source to a constant level (set an extinction ratio to a constant level) (for example, Patent Document 1).
On the other hand, to overcome degradation of an electric waveform generated due to the degradation of an FET inside the IC, there is also disclosed a method of adjusting a crosspoint of an electronic waveform by controlling the extinction ratio of the EA modulator output light to a constant level (for example, Patent Document 2).
Patent Document 1: Japanese Patent Application Laid-open No. 2000-36794
Patent Document 2: Japanese Patent Application Laid-open No. 2001-189699